Cyclical deposition of germanium

ABSTRACT

In some aspects, methods for forming a germanium thin film using a cyclical deposition process are provided. In some embodiments, the germanium thin film is formed on a substrate in a reaction chamber, and the process includes one or more deposition cycles of alternately and sequentially contacting the substrate with a vapor phase germanium precursor and a nitrogen reactant. In some embodiments, the process is repeated until a germanium thin film of desired thickness has been formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 15/435,769, filed Feb. 17, 2017, which is a continuation of U.S. application Ser. No. 14/969,413, filed Dec. 15, 2015, issued as U.S. Pat. No. 9,576,794, which is a continuation of U.S. application Ser. No. 14/135,383, filed Dec. 19, 2013, issued as U.S. Pat. No. 9,218,963, each of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The application relates to deposition processes for forming germanium films.

Background

Germanium thin films are used in a variety of contexts, particularly in the semiconductor industry. For example, Ge devices are of interest because of Ge high hole mobility. Low D_(it) interface formation with HfO₂ high-k material will allow for good Ge based FinFETs. Germanium films may also be used for epitaxial layers, sacrificial layers and for the formation of germanides. In many situations, the films are deposited on high aspect ratio structures, such as in the formation of FinFETs. Deposition of relatively pure germanium at relatively low temperatures by highly conformal processes has heretofore been difficult.

SUMMARY

In some aspects, methods for forming a germanium thin film using a cyclical deposition process are provided. In some embodiments, the germanium thin film is formed on a substrate in a reaction chamber, and the process includes one or more deposition cycles of alternately and sequentially contacting the substrate with a vapor phase germanium precursor and a nitrogen reactant. In some embodiments, the process is repeated until a germanium thin film of desired thickness has been formed.

According to some embodiments, after contacting the substrate with a vapor phase germanium precursor, the substrate is exposed to a purge gas and/or a vacuum to remove excess germanium precursor and reaction byproducts from the substrate surface, if any. In some embodiments, after contacting the substrate with a vapor phase nitrogen reactant, the substrate is exposed to a purge gas and/or a vacuum to remove excess nitrogen reactant and reaction byproducts from the substrate surface, if any.

According to some embodiments, the germanium reactant at least partially decomposes in each deposition cycle. In some embodiments, the process is performed at a temperature below the temperature at which the germanium precursor decomposes without the presence of another precursor. In some embodiments, the process is performed at a temperature below about 600° C. In some embodiments, the process is performed at a temperature below about 500° C.

According to some embodiments, the germanium film is an elemental germanium film. In some embodiments, the germanium film comprises less than about 5 at-% impurities. In some embodiments, the germanium thin film contains less than about 3 at-% oxygen. The process of Claim 1, wherein the growth rate is greater than about 2 angstrom s/cycle.

According to some embodiments, the nitrogen reactant comprises ammonia, elemental nitrogen, nitrogen plasma, or nitrogen radicals. In some embodiments, the Ge-precursor is a germanium alkoxide or alkylamine. In some embodiments, the germanium precursor is not a germane. In some embodiments, the germanium precursor is Ge(OCH₂CH₃)₄.

In some aspects, methods for forming a germanium thin film using a cyclical deposition process are provided, in which the cyclical process includes continuously flowing a nitrogen reactant through the reaction chamber, contacting the substrate with a vapor phase Ge precursor, removing excess Ge precursor and reaction by products, if any, from the reaction space by ceasing to provide Ge precursor to the reaction chamber, and repeating the contacting and removing steps until a germanium thin film of the desired thickness has been formed. In some embodiments, the concentration of the nitrogen reactant is kept low enough to suppress any gas-phase reaction between the nitrogen reactant and the Ge precursor.

In some aspects, methods for forming a germanium thin film using a cyclical deposition process are provided, in which at least one of the deposition cycles includes contacting the substrate with a first vapor phase germanium precursor, exposing the substrate to a purge gas and/or a vacuum to remove excess germanium precursor and reaction by-products from the substrate surface, if any, contacting the substrate with a second vapor phase reactant, exposing the substrate to a purge gas and/or a vacuum to remove excess nitrogen reactant and reaction by-products from the substrate surface, if any, and repeating the contacting and removing steps until a germanium thin film of the desired thickness has been formed. In some embodiments, the second reactant does not comprise H₂ or H₂-based plasma species. In some embodiments, the germanium thin film comprises elemental germanium with less than 5% impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will be better understood from the appended drawings, which are meant to illustrate and not to limit the invention, and wherein:

FIG. 1 illustrates an exemplary cyclical germanium deposition process according to some embodiments of the present disclosure.

FIGS. 2A-D show images from a scanning electron microscope (SEM) of various germanium films deposited at different temperatures.

FIGS. 3A and 3B show SEM images of two germanium films deposited at 375° C. and 385° C., respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In one aspect, methods of depositing germanium thin films by a cyclical deposition process are provided. In some embodiments a germanium thin film is formed on a substrate by a vapor deposition process comprising one or more deposition cycles in which a substrate is alternately and sequentially contacted with a vapor phase germanium precursor and a vapor phase reducing agent, typically a nitrogen reactant. In the first part of the deposition cycle, a layer of the germanium precursor forms on the substrate surface. Iii the second part of the deposition cycle, the nitrogen reactant subsequently reacts with the germanium precursor to form a germanium thin film.

In some embodiments the vapor deposition process is an atomic layer deposition process and the germanium precursor does not decompose. However, in some embodiments the germanium precursor at least partially decomposes during the deposition process.

In some embodiments a relatively pure germanium film is deposited by a cyclical deposition process. For example, the germanium film may be at least 90%, 95%, 97%, 98% or 99% pure germanium in some embodiments.

In some embodiments the germanium precursor may be an alkoxide. For example, in some embodiments the germanium precursor is selected from germanium ethoxide (GeOEt)₄ and tetrakis(dimethylamino) germanium (TDMAGe). Other possible germanium precursors are provided below and may include germanium compounds containing Ge—O bonds, Ge—C bonds (for example, germanium alkyls), or Ge—N bonds (for example, germanium alkylamines). In some embodiments the germanium precursor is not a halide. In some embodiments, the Ge precursor contains a halide in at least one ligand, but not in all ligands. According to some embodiments, the germanium precursor does not include only germanium and hydrogen. For example, in some embodiments the germanium precursor is not a germane (GeH_(x)).

In some embodiments the nitrogen reactant comprises compounds containing N—H bonds, such as NH₃, nitrogen-containing plasma, atomic nitrogen, and/or nitrogen radicals

In some embodiments, germanium thin films are deposited by alternately and sequentially contacting a substrate with Ge(OCH₂CH₃)₄ and NH₃. In some embodiments germanium thin films are deposited by alternately and sequentially contacting a substrate with tetrakis(dimethylamino) germanium (TDMAGe) and NH₃.

Germanium films may be used in a variety of different contexts. For example in some embodiments a germanium film may serve as an epitaxial layer. Iii some embodiments a germanium film does not serve as an epitaxial or single-crystal layer. In some embodiments a germanium film may serve as a sacrificial layer. In some embodiments a germanium layer may be used for the formation of a germanide. In some embodiments, a germanium layer may be used in a germanium condensation process. In some embodiments a germanium layer may be deposited on a high aspect ratio structure, such as a FinFET structure. Other contexts in which germanium thin films may be utilized will be apparent to the skilled artisan. For example, germanium thin films may find use in optical applications. In some embodiments, the germanium films are annealed after the deposition as desired depending on the application.

The cyclical deposition processes disclosed herein allow for conformal deposition of germanium films. In some embodiments, the germanium films deposited have at least 50%, 80%, 90%, 95% or higher conformality. In some embodiments the films are about 100% conformal.

The substrate may be, for example, a semiconductor substrate. In some embodiments the substrate surface is H-terminated.

The substrate may be treated prior to depositing the germanium layer. For example, the substrate may be treated with a passivation chemical to prevent oxidation during air exposure prior to depositing germanium. In other embodiments the substrate is treated to form an interfacial layer prior to depositing germanium.

In some embodiments, following germanium deposition, a further film is deposited. The additional film may be directly over and contacting the germanium layer. In some embodiments a metal film is deposited over the germanium film, for example for forming a metal germanide. Nickel may be deposited over a germanium layer and subsequently annealed to form a nickel germanide. In some embodiments a high-k film is deposited after the germanium is deposited. The high-k layer or other film may be deposited by ALD or by other known deposition methods. In some embodiments, a HfO₂ layer is deposited over the germanium layer. In some embodiments an Al₂O₃ layer is deposited over the germanium layer. In some embodiments, a deposition process for a film deposited on top of a germanium film uses water as an oxygen source. In some embodiments, a deposition process for a film deposited on top of a germanium film uses ozone as an oxygen source. In some embodiments a deposition process for a film deposited on top of a germanium film uses oxygen atoms, oxygen radicals or oxygen containing plasma as an oxygen source.

Cyclical Deposition Process

As noted above, processes described herein enable use of cyclical layer deposition techniques to deposit conformal germanium layers. The cyclical deposition process is mostly surface-controlled (based on controlled reactions at the substrate surface) and thus has the advantage of providing high conformality at relatively low temperatures. However, in some embodiments, the germanium precursor may at least partially decompose. Accordingly, in some embodiments the cyclical process described herein is a pure ALD process in which no decomposition of precursors is observed. In other embodiments, reaction conditions, such as reaction temperature, are selected such that at least some decomposition takes place.

Cyclical deposition processes are based on alternatingly providing vapor phase reactants to a reaction space to interact with a substrate surface contained therein. Gas phase reactions are avoided by feeding the precursors alternately and sequentially into the reaction chamber. Vapor phase reactants may be separated from each other in the reaction chamber, for example, by removing excess reactants and/or reactant by-products from the reaction chamber between reactant pulses. Removal may occur through the use of a purge gas and/or an applied vacuum.

Briefly, a substrate is loaded into a reaction chamber and is heated to a suitable deposition temperature, generally at lowered pressure. Deposition temperatures may be maintained above the temperature at which the germanium precursor does not decompose in the presence of another reactant and below the germanium precursor's thermal decomposition temperature. The temperature should also be at a high enough level to provide the activation energy for the desired surface reactions. Because of the variability in decomposition temperatures of different compounds, the actual reaction temperature in any given embodiment may be selected based on the specifically chosen precursors. In some embodiments the deposition temperature is below about 600° C. In some embodiments, the deposition temperature is below about 500° C. In some embodiments the deposition temperature is below about 450° C. In some embodiments the deposition temperature is preferably below about 400° C. and even, in some cases, below about 375° C.

A first germanium reactant is conducted into the chamber in the form of vapor phase pulse and contacted with the surface of a substrate. In some embodiments the substrate surface comprises a three dimensional structure. In some embodiments, conditions are preferably selected such that more than about one monolayer of the germanium precursor is adsorbed. Excess first reactant and reaction byproducts, if any, may be removed from the substrate and substrate surface and from proximity to the substrate and substrate surface. In some embodiments reactant and reaction byproducts, if any, may be removed by purging. Purging may be accomplished, for example, with a pulse of inert gas such as nitrogen or argon.

Purging the reaction chamber means that vapor phase precursors and/or vapor phase byproducts are removed from the reaction chamber such as by evacuating the chamber with a vacuum pump and/or by replacing the gas inside the reactor with an inert gas such as argon or nitrogen. Typical purging times are from about 0.05 seconds to about 20 seconds, more preferably between about 1 second and about 10 seconds, and still more preferably between about 1 second and about 2 seconds. However, other purge times can be utilized if necessary, such as when depositing layers over extremely high aspect ratio structures or other structures with complex surface morphology. The appropriate purge times can be readily determined by the skilled artisan based on the particular circumstances.

Another method for removing excess reactants—metal precursors or nitrogen reactants, reaction byproducts, etc.—from the substrate surface or from the area of the substrate may involve physically moving the substrate from a location containing the reactant and/or reaction byproducts.

A second gaseous reactant is pulsed into the chamber where it reacts with the first germanium reactant on the surface to form essentially pure germanium. The second reactant is a reducing agent that removes ligands from the first reactant on the substrate surface. In some embodiments the second reactant is a nitrogen reactant. Excess second reactant and gaseous by-products of the surface reaction are removed from the substrate, for example by purging them out of the reaction chamber, preferably with the aid of an inert gas. The steps of pulsing and removing are repeated until a thin film of germanium of the desired thickness has been formed on the substrate, with each cycle typically leaving more than about a molecular monolayer.

As mentioned above, in some embodiments each pulse or phase of each cycle may be self-limiting. An excess of reactant precursors is supplied in each phase to saturate the susceptible structure surfaces. Surface saturation ensures reactant occupation of all available reactive sites (subject, for example, to physical size or “steric hindrance” restraints) and thus ensures excellent step coverage. In some arrangements, the degree of self-limiting behavior can be adjusted by adjusting the reaction temperature to allow for some decomposition of the germanium reactant in combination with the nitrogen reactant.

In some embodiments the second nitrogen reactant can be provided continuously throughout the deposition process. Thus, in some embodiments a nitrogen reactant is flowed continuously throughout the deposition process and a germanium precursor is provided at regular intervals. In some embodiments nitrogen reactant is flowed continuously at a low concentration. The concentration is kept low enough to prevent gas-phase reactions between the germanium precursor and the nitrogen reactant thereby maintaining the surface-controlled nature of the process.

In some embodiments, a reaction space can be in a single-wafer reactor or a batch reactor where deposition on multiple substrates takes place at the same time. In some embodiments the substrate on which deposition is desired, such as a semiconductor workpiece, is loaded into a reactor. The reactor may be part of a cluster tool in which a variety of different processes in the formation of an integrated circuit are carried out. In some embodiments a flow-type reactor is utilized. In some embodiments a high-volume manufacturing-capable single wafer reactor is used. In other embodiments a batch reactor comprising multiple substrates is used. For embodiments in which batch reactors are used, the number of substrates may be in the range of 10 to 200, in the range of 50 to 150, or in the range of 100 to 130.

According to some embodiments, a showerhead reactor may be used.

Examples of suitable reactors that may be used include commercially available equipment such as the F-120® reactor, F-450® reactor, Pulsar® reactors—such as the Pulsar® 2000 and the Pulsar® 3000-EmerALD® reactor and Advance® 400 Series reactors, available from ASM America, Inc of Phoenix, Ariz. and ASM Europe B.V., Almere, Netherlands. Other commercially available reactors include those from ASM Japan K.K (Tokyo, Japan) under the tradename Eagle® XP and XP8. In addition to these reactors, many other kinds of reactors capable of growth of thin films, including CVD reactors equipped with appropriate equipment and means for pulsing the precursors can be employed.

Suitable batch reactors include, but are not limited to, reactors commercially available from and ASM Europe B.V (Almere, Netherlands) under the trade names ALDA400™ and A412™. In some embodiments a vertical batch reactor is utilized in which the boat rotates during processing, such as the A412™. Thus, in some embodiments the wafers rotate during processing. In some embodiments in which a batch reactor is used, wafer-to-wafer uniformity is less than 3% (1 sigma), less than 2%, less than 1% or even less than 0.5%.

The cyclical germanium processes described herein can optionally be carried out in a reactor or reaction space connected to a cluster tool. In a cluster tool, because each reaction space is dedicated to one type of process, the temperature of the reaction space in each module can be kept constant, which improves the throughput compared to a reactor in which the substrate is heated up to the process temperature before each run.

Referring to FIG. 1, and according to some embodiments, a germanium thin film is formed by a cyclical deposition process 100 comprising multiple pulsing cycles, each cycle comprising:

-   -   providing a pulse of a vaporized first Ge precursor into the         reaction chamber at step 120 to contact the substrate surface         with the Ge precursor;     -   removing excess Ge precursor and reaction by products, if any,         at step 130;     -   providing a pulse of a second nitrogen reactant into the         reaction chamber at step 140,     -   removing at step 150 excess second reactant and any gaseous         by-products formed in the reaction between the Ge precursor         layer on the first surface of the substrate and the second         reactant, and     -   repeating at step 160 the pulsing and removing steps until a         germanium thin film of the desired thickness has been formed.

As mentioned above, in some embodiments the substrate may be pretreated prior to beginning the deposition process 100. In FIG. 1 this is indicated by step 110 in which the substrate is optionally subjected to a pretreatment process.

When the Ge precursor contacts the substrate, the Ge precursor may form at least a monolayer, less than a monolayer, or more than a monolayer.

In some embodiments, a carrier gas is flowed continuously to the reaction space throughout the deposition process. In each deposition cycle the first germanium precursor is pulsed into the reaction chamber. Excess germanium precursor is then removed from the reaction chamber. In some embodiments, the carrier gas comprises nitrogen. In some embodiments a separate purge gas is utilized.

The Ge precursor employed in the cyclical processes may be solid, or gaseous material under standard conditions (room temperature and atmospheric pressure), provided that the Ge precursor is in vapor phase before it is conducted into the reaction chamber and contacted with the substrate surface.

“Pulsing” a vaporized reactant into the reaction chamber means that the precursor vapor is conducted into the chamber for a limited period of time. Typically, the pulsing time is from about 0.05 seconds to about 10 seconds. However, depending on the particular circumstances, including for example the substrate type and its surface area, the pulsing time may be even higher than about 10 seconds.

In some embodiments, for example for a 300 mm wafer in a single wafer reactor, the Ge precursor is pulsed for from about 0.05 seconds to about 10 seconds, for from about 0.1 seconds to about 5 seconds or from about 0.3 seconds to about 3.0 seconds.

The nitrogen reactant may be pulsed for from about 0.05 seconds to about 10 seconds, from about 0.1 seconds to about 5 seconds, or for from about 0.2 seconds to about 3.0 seconds. However, pulsing times for one or both reactants can be on the order of minutes in some cases. The optimum pulsing time for each reactant can be determined by the skilled artisan based on the particular circumstances.

As mentioned above, in some embodiments the Ge precursor is a germanium alkoxide, for example Ge(OEt)₄ or Ge(OMe)₄. In some embodiments, the Ge precursor is TDMAGe. In some embodiments, the Ge precursor includes alkyl and/or alkylamine groups. In some embodiments the Ge-precursor is not a halide. In some embodiments the Ge-precursor may comprise a halogen in at least one ligand, but not in all ligands. The germanium precursor may be provided with the aid of an inert carrier gas, such as argon.

In some embodiments the nitrogen reactant comprises a nitrogen-hydrogen bond. In some embodiments the nitrogen reactant is ammonia (NH₃). In some embodiments, the nitrogen reactant is molecular nitrogen. In some embodiments the nitrogen reactant is a nitrogen containing plasma. In some embodiments, the nitrogen source comprises an activated or excited nitrogen species. The nitrogen reactant may be a provided in a nitrogen-containing gas pulse that can be a mixture of nitrogen reactant and inactive gas, such as argon.

In some embodiments, a nitrogen-containing plasma is formed in the reactor. In some embodiments, the plasma may be formed in situ on top of the substrate or in close proximity to the substrate. In other embodiments, the plasma is formed upstream of the reaction chamber in a remote plasma generator and plasma products are directed to the reaction chamber to contact the substrate. As will be appreciated by the skilled artisan, in the case of remote plasma, the pathway to the substrate can be optimized to maximize electrically neutral species and minimize ion survival before reaching the substrate.

Irrespective of the nitrogen reactant used, in some embodiments of the present disclosure, the use of a nitrogen reactant does not contribute significant amounts of nitrogen to the deposited film. According to some embodiments, the resulting germanium film contains less than about 5-at %, less than about 2-at % or even less than about 1-at % nitrogen. In some embodiments, the nitrogen content of the germanium film is less than about 0.5-at % or even less than about 0.2-at %.

In some embodiments hydrogen reactants are not used in the deposition process. In some embodiments, elemental hydrogen (H₂) is not provided in at least one deposition cycle, or in the entire deposition process. In some embodiments, hydrogen plasma is not provided in at least one deposition cycle or in the entire deposition process. In some embodiments, hydrogen atoms or radicals are not provided in at least one deposition cycle, or in the entire deposition process.

In some embodiments the Ge precursor comprises at least one amine or alkylamine ligand, such as those presented in formulas (2) through (6) and (8) and (9), and the nitrogen reactant comprises NH₃.

Before starting the deposition of the film, the substrate is typically heated to a suitable growth temperature, as discussed above. The preferred deposition temperature may vary depending on a number of factors such as, and without limitation, the reactant precursors, the pressure, flow rate, the arrangement of the reactor, and the composition of the substrate including the nature of the material to be deposited on. In some embodiments the deposition temperature is selected to be between the temperature where the germanium precursor does not decompose without the second nitrogen precursor, at the lower end, and the temperature where the precursor does decompose by itself, at the upper end. As discussed elsewhere, in some embodiments the temperature may be less than about 600° C., less than about 450° C., less than about 400° C., and in some cases, less than about 375° C. In some embodiments using Ge(OCH₂CH₃)₄ and NH₃ as the germanium and nitrogen reactants, the temperature is about 350° C.

The processing time depends on the thickness of the layer to be produced and the growth rate of the film. In ALD, the growth rate of a thin film is determined as thickness increase per one cycle. One cycle consists of the pulsing and removing steps of the precursors and the duration of one cycle is typically between about 0.2 seconds and about 30 seconds, more preferably between about 1 second and about 10 seconds, but it can be on order of minutes or more in some cases, for example, where large surface areas and volumes are present.

In some embodiments the growth rate of the germanium thin films may be greater than or equal to about 2 Å/cycle, greater than or equal to about 5 Å/cycle, greater than or equal to about 10 Å/cycle, and, in some embodiments, even greater than about 15 Å/cycle.

In some embodiments the germanium film formed is a relatively pure germanium film. Preferably, aside from minor impurities no other metal or semi-metal elements are present in the film. In some embodiments the film comprises less than 1-at % of metal or semi-metal other than Ge. In some embodiments, the germanium film comprises less than about 5-at % of any impurity other than hydrogen, preferably less than about 3-at % of any impurity other than hydrogen, and more preferably less than about 1-at % of any impurity other than hydrogen. In some embodiments a germanium film comprises less than about 5 at-% nitrogen, less than about 3 at-% nitrogen less than about 2 at-% nitrogen or even less than about 1 at-% nitrogen. In some embodiments, a pure germanium film comprises less than about 2-at % oxygen, preferably less than about 1-at % or less than about 0.5-at % and even less than about 0.25-at %.

In some embodiments a germanium precursor comprising oxygen is utilized and the germanium film comprises no oxygen or a small amount of oxygen as an impurity. In some embodiments the germanium film deposited using a germanium precursor comprising oxygen may comprise less than about 2 at-% oxygen, less than about 1 at-%, less than about 0.5 at-% or even less than about 0.25 at-%.

In some embodiments, the germanium film formed has step coverage of more than about 50%, more than about 80%, more than about 90%, or even more than about 95% on structures which have high aspect ratios. In some embodiments high aspect ratio structures have an aspect ratio that is more than about 3:1 when comparing the depth or height to the width of the feature. In some embodiments the structures have an aspect ratio of more than about 5:1, or even an aspect ratio of 10:1 or greater.

Ge Precursors

A number of different Ge precursors can be used in the cyclical processes. In some embodiments the Ge precursor is tetravalent (i.e. Ge has an oxidation state of +IV). In some embodiments, the Ge precursor is not divalent (i.e., Ge has an oxidation state of +II). In some embodiments, the Ge precursor may comprise at least one alkoxide ligand. In some embodiments, the Ge precursor may comprise at least one amine or alkylamine ligand. In some embodiments the Ge precursor is a metal-organic or organometallic compound. In some embodiments the Ge precursor comprises at least one halide ligand. In some embodiments the Ge precursor does not comprise a halide ligand.

In some embodiments the Ge precursor comprises a Ge—O bond. In some embodiments the Ge precursor comprises a Ge—N bond. In some embodiments the Ge precursor comprises a Ge—C bond. In some embodiments the Ge precursor does not comprise Ge—H bond. In some embodiments the Ge precursor comprises equal or less than two Ge—H bonds per one Ge atom.

In some embodiments the Ge precursor is not solid at room temperature (e.g., about 20° C.).

For example, Ge precursors from formulas (1) through (9) below may be used in some embodiments. GeOR₄  (1)

Wherein R is can be independently selected from the group consisting of alkyl and substituted alkyl; GeR_(x)A_(4-x)  (2)

Wherein the x is an integer from 1 to 4;

R is an organic ligand and can be independently selected from the group consisting of alkoxides, alkyl silyls, alkyl, substituted alkyl, alkylamines; and

A can be independently selected from the group consisting of alkyl, substituted alkyl, alkoxides, alkylsilyls, alkyl, alkylamines, halide; and hydrogen. Ge(OR)_(x)A_(4-x)  (3)

Wherein the x is an integer from 1 to 4;

R can be independently selected from the group consisting of alkyl and substituted alkyl; and

A can be independently selected from the group consisting of alkyl, alkoxides, alkylsilyls, alkyl, substituted alkyl, alkylamines, halide, and hydrogen. Ge(NR^(I)R^(II))₄  (4)

Wherein R^(I) can be independently selected from the group consisting of hydrogen, alkyl and substituted alkyl; and

R^(II) can be independently selected from the group consisting of alkyl and substituted alkyl; Ge(NR^(I)R^(II))A_(4-x)  (5)

Wherein the x is an integer from 1 to 4;

R^(I) can be independently selected from the group consisting of hydrogen, alkyl and substituted alkyl; and

R^(II) can be independently selected from the group consisting of alkyl and substituted alkyl;

A can be independently selected from the group consisting of alkyl, alkoxides, alkylsilyls, alkyl, substituted alkyl; alkylamines, halide, and hydrogen. Ge_(n)(NR^(I)R^(II))_(2n+2)  (6)

Wherein the n is an integer from 1 to 3;

R^(I) can be independently selected from the group consisting of hydrogen; alkyl and substituted alkyl; and

R^(II) can be independently selected from the group consisting of alkyl and substituted alkyl; Ge_(n)(OR)_(2n+2)  (7)

Wherein the n is an integer from 1 to 3; and

Wherein R can be independently selected from the group consisting of alkyl and substituted alkyl; Ge_(n)R_(2n+2)  (8)

Wherein the n is an integer from 1 to 3; and

R is an organic ligand and can be independently selected from the group consisting of alkoxides, alkylsilyls, alkyl, substituted alkyl, alkylamines. A_(3-x)R_(x)Ge-GeR_(y)A_(3-y)  (9)

Wherein the x is an integer from 1 to 3;

y is an integer from 1 to 3;

R is an organic ligand and can be independently selected from the group consisting of alkoxides, alkylsilyls, alkyl, substituted alkyl, alkylamines; and

A can be independently selected from the group consisting of alkyl, alkoxides, alkylsilyls, alkyl, substituted alkyl, alkylamines, halide, and hydrogen.

Preferred options for R include, but are not limited to, methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, tertbutyl for all formulas, more preferred in ethyl and methyl. In some embodiments, the preferred options for R include, but are not limited to, C₃-C₁₀ alkyls, alkenyls, and alkynyls and substituted versions of those, more preferably C₃-C₆ alkyls, alkenyls, and alkenyls and substituted versions of those.

In some embodiments the Ge precursor comprises one or more halides. For example, the precursor may comprise 1, 2, or 3 halide ligands. However, as mentioned above, in some embodiments the Ge precursor does not comprise a halide.

In some embodiments a germane (GeH_(x)) is not used.

In some embodiments alkoxide Ge precursors may be used, including, but are not limited to, one or more of Ge(OMe)₄, Ge(OEt)₄, Ge(O^(i)Pr)₄, Ge(O^(n)Pr)₄ and Ge(O^(t)Bu)₄. In some embodiments the Ge precursor is TDMAGe. In some embodiments the Ge precursor is TDEAGe. In some embodiments the Ge precursor is TEMAGe.

EXAMPLES

The ability to deposit germanium films by decomposing germanium precursors at relatively low temperatures was tested. At temperatures of about 375° C. and about 400° C., no germanium films were seen when only alternating Ge precursor pulses and carrier gas purges without second reactant. In these decomposition tests, Ge(OEt)₄ was used as the germanium precursor and was kept at room temperature. The Ge(OEt)₄ was cyclically pulsed through the reaction space for 3 seconds followed by a 6 second purge.

However, by utilizing a cyclical deposition process in which the substrate was alternately contacted with a germanium precursor and a nitrogen reactant, deposition was observed in the temperature range of about 350° C. to about 400° C.

In one set of experiments, germanium films were deposited in an F-450® reactor at 350° C. by repeating a deposition cycle comprising alternate and sequential pulses of Ge(OEt)₄ and NH₃. The Ge(OEt)₄ was kept at room temperature. The NH₃ was flowed through the reaction space at 100 sccm. Each cycle comprised a 3 seconds germanium precursor pulse, a 6 second purge, and a 3 second nitrogen reactant pulse followed by a 10 second purge. A thick metal-like film of greater than 80 nm was formed with 2000 cycles.

Germanium films were also deposited in an Pulsar® 2000 reactor at temperatures ranging from about 365° C. to about 400° C. using germanium ethoxide (Ge(OEt)₄) as the Ge precursor, and ammonia (NH₃) as the nitrogen reactant. Ge(OEt)₄ is a liquid with a vapor pressure of about 0.2 MIT at 55° C. The Ge(OEt)₄ was kept at room temperature. The NH₃ was flowed through the reaction space at 100 sccm. Each cycle comprised a 3 seconds germanium precursor pulse, a 5 second purge, and a 3 second nitrogen reactant pulse followed by a 5 second purge. Each film was deposited using a cyclical process of 500 cycles. Composition was determined by Rutherford backscattering spectroscopy (“RBS”). The results are summarized in Table 1 below.

TABLE 1 Germanium films achieved with cyclical process. Atomic Concentration (at %) Sample Deposition T (° C.) Thickness (nm) Ge O C N 1 365 162 95.5 1.5 1.6 1.4 2 375 375 97.8 0.8 0.9 0.5 3 385 493 98.6 0.5 0.7 0.2 4 400 921 99.2 0.2 0.5 0.1

FIGS. 2A-D show SEM images of the four germanium films summarized in Table 1 above. Accordingly, FIG. 2A is an image of the film deposited at a temperature of about 365° C.; FIG. 2B is an image of the film deposited at a temperature of about 375° C.; FIG. 2C is an image of the film deposited at a temperature of about 385° C.; and FIG. 2D is an image of the film deposited at a temperature of about 400° C.

FIGS. 3A and 3B show SEM images of two of the germanium films summarized in Table 1. The image in FIG. 3A is of the film deposited at a reaction temperature of about 375° C., and the image in FIG. 3B is of the film deposited at a reaction temperature of about 385° C. Consistent with the data of Table 1, FIGS. 3A and 3B demonstrate that there is a temperature dependence for the growth rate, because the film deposited at about 385° C. is thicker than the film deposited at about 375° C. even though each film was formed using a cyclical process of 500 cycles.

The deposited germanium films exhibited a relatively high sheet resistance of about 4000-9000 Ω/sq.

Although certain embodiments and examples have been discussed, it will be understood by those skilled in the art that the scope of the claims extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and obvious modifications and equivalents thereof. 

What is claimed is:
 1. A process for forming an elemental germanium thin film on a substrate in a reaction chamber, the process comprising multiple deposition cycles comprising alternately and sequentially contacting the substrate with a vapor phase germanium precursor and a vapor phase reactant which is not hydrogen, wherein the vapor phase reactant which is not hydrogen reacts with the vapor phase germanium precursor to form elemental germanium.
 2. The process of claim 1 further comprising, after contacting the substrate with the vapor phase germanium precursor, removing excess vapor phase germanium precursor from the substrate surface prior to contacting the substrate with the vapor phase reactant.
 3. The process of claim 2, wherein removing comprises exposing the substrate to a purge gas.
 4. The process of claim 1 further comprising, after contacting the substrate with the vapor phase reactant, removing excess vapor phase reactant from the substrate surface.
 5. The process of claim 1, wherein the germanium reactant at least partially decomposes in each deposition cycle.
 6. The process of claim 5, wherein the process is performed at a temperature below the temperature at which the germanium precursor decomposes in the absence of another precursor.
 7. The process of claim 1, wherein the germanium thin film comprises less than about 5 at-% impurities.
 8. The process of claim 1, wherein the vapor phase reactant comprises nitrogen.
 9. The process of claim 1, wherein the vapor phase reactant comprises ammonia, elemental nitrogen, nitrogen plasma, or nitrogen radicals.
 10. The process of claim 1, wherein the vapor phase germanium precursor comprises a germanium halide, germanium alkoxide or germanium alkylamine.
 11. The process of claim 1, wherein the germanium precursor is tetravalent.
 12. The process of claim 1, wherein the germanium precursor comprises a halogen in at least one ligand but not all ligands.
 13. The process of claim 1, wherein the germanium precursor is not a germane.
 14. The process of claim 1, wherein the germanium precursor is Ge(OCH₂CH₃)₄.
 15. The process of claim 1, wherein the process is performed at a temperature below about 500° C.
 16. The process of claim 1, wherein the vapor phase reactant flows continuously through the reaction chamber.
 17. The process of claim 16, wherein the concentration of the vapor phase reactant is kept low enough to suppress any gas-phase reaction between the vapor phase reactant and the vapor phase germanium precursor.
 18. A method for depositing an elemental germanium thin film on a surface of a substrate in a reaction chamber the method comprising: conducting a deposition cycle comprising, in order: contacting the substrate surface with a first vapor phase tetravalent germanium precursor; exposing the substrate to a purge gas and/or a vacuum; contacting the substrate surface with a second vapor phase reactant which is not hydrogen and does not comprise H₂ or H₂-based plasma species, wherein the second vapor phase reactant reacts with the first vapor phase tetravalent germanium precursor on the substrate surface to form elemental germanium; and exposing the substrate to a purge gas and/or a vacuum; and repeating the deposition cycle to form an elemental germanium thin film on the surface of the substrate.
 19. The method of claim 18, wherein the germanium precursor is a germanium halide, germanium alkylamine or germanium alkoxide.
 20. The process of claim 18, wherein the second vapor phase reactant comprises nitrogen. 